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Transcript
Essentials of Geology David Sallee Earthquakes and Earth’s Interior Chapter 14 An aerial view of the San Andreas fault in the Carrizo Plain, Central California What is the Elastic Rebound Theory? What are Earthquakes? The shaking or trembling caused by the sudden release of energy Usually associated with faulting or breaking of rocks Continuing adjustment of position results in aftershocks Explains how energy is stored in rocks • Rocks bend until the strength of the rock is exceeded • Rupture occurs and the rocks quickly rebound to an undeformed shape • Energy is released in waves that radiate outward from the fault What is Seismology? What is Seismology? The Focus and Epicenter of an Earthquake The point within Earth where faulting begins is the focus, or hypocenter The point directly above the focus on the surface is the epicenter Seismographs record earthquake events At convergent boundaries, boundaries, focal depth increases along a dipping seismic zone called a Benioff zone 1 Where Do Earthquakes Occur and How Often? Where do Earthquakes Occur and How Often? ~80% of all earthquakes occur in the circumcircum-Pacific belt • most of these result from convergent margin activity • ~15% occur in the MediterraneanMediterranean-Asiatic belt • remaining 5% occur in the interiors of plates and on spreading ridge centers • more than 150,000 quakes strong enough to be felt are recorded each year Damage in Oakland, CA, 1989 What are Seismic Waves? What are Seismic Waves? Response of material to the arrival of energy fronts released by rupture Two types: Body waves Surface waves Body waves • P or primary waves fastest waves travel through solids, liquids, or gases compressional wave, material movement is in the same direction as wave movement • S or secondary waves • P and S slower than P waves travel through solids only shear waves - move material perpendicular to wave movement • R and L How is an Earthquake’s Epicenter Located? What are Seismic Waves? Surface Waves • Travel just below or along the ground’s surface • Slower than body waves; rolling and sideside-toto-side movement • Especially damaging to buildings Seismic wave behavior • P waves arrive first, then S waves, then L and R • Average speeds for all these waves is known • After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter. 2 How is an Earthquake’s Epicenter Located? TimeTime-distance graph showing the average travel times for PP- and SSwaves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the PPand SS- waves How is the Size and Strength of an Earthquake Measured? Modified Mercalli Intensity Map • 1994 Northridge, CA earthquake, magnitude 6.7 Intensity How is an Earthquake’s Epicenter Located? How is the Size and Strength of an Earthquake Measured? Ground Shaking • amplitude, duration, and damage increases in poorly consolidat ed rocks Magnitude • Richter scale measures total amount of energy released by an earthquake; independent of intensity • Amplitude of the largest wave produced by an event is corrected for distance and assigned a value on an openopen-ended logarithmic scale • subjective measure of the kind of damage done and people’s reactions to it • isoseismal lines identify areas of equal intensity What are the Destructive Effects of Earthquakes? What are the Destructive Effects of Earthquakes? Three seismograph stations are needed to locate the epicenter of an earthquake A circle where the radius equals the distance to the epicenter is drawn The intersection of the circles locates the epicenter Building collapse Fire Tsunami Ground failure 3 Can Earthquakes be Predicted? Can Earthquakes be Predicted? Earthquake Precursors • changes in elevation or tilting of land surface, fluctuations in groundwater levels, magnetic field, electrical resistance of the ground • seismic dilatancy model • seismic gaps • include laboratory and field studies of rocks before, during, and and after earthquakes • monitor activity along major faults • produce risk assessments Can Earthquakes be Controlled? What is Earth’s Interior Like? A profile showing seismic velocities versus depth • several discontinuities indicate changes in Earth materials or their properties • discontinuities are the basis for subdividing Earth’s interior into concentric layers Crust, mantle, core • behavior and travel times of P and S waves helps define interior structure • velocity of waves is dependent on the density and elasticity of material they travel through • waves are bent (refracted) or bounced (reflected) as they pass through different materials in Earth Graph showing the relationship between the amount of waste injected into wells per month and the average number of Denver earthquakes per month Some have suggested that pumping fluids into seismic gaps will cause small earthquakes while preventing large ones What is Earth’s Interior Like? Earthquake Prediction Programs The Core Density and Composition of the Core • behavior of P and S waves indicates a solid inner and liquid outer core • inner core is iron/nickel, rotates more rapidly than outer core • outer core is iron mixed with sulfur, density of 9.9 to 12.3 gm/cm3 4 The Mantle The Mantle Structure and Composition of the Mantle • Seismic wave velocities generally increase with depth, but several other discontinuities exist • The lowlow-velocity zone is inferred to represent zones of partial melting in the asthenosphere • Composition believed to be that of the igneous rock peridotite Discontinuities • Sharp velocity increase in wave travel times at a depth of about 30km - called the Moho • The Moho separates the crust from the mantle Seismic Tomography ThreeThree-dimensional models of Earth’s interior • More complete analysis of seismic waves indicated hot and cold areas within the mantle • Hot areas underlie spreading centers and volcanic areas • Cold areas underlie the older interior portions of the large continents The Crust Continental crust • Overall composition similar to granite • Low density (2(2-3gm/cm²) • Averages 35km thick, more under mountain ranges Oceanic crust • Gabbro/basalt composition • Higher density (3gm/cm²) • Between 5 and 10 km thick Earth’s Internal Heat Geothermal gradient • Temperature increases with depth • Averages 25°C/km • Gradient is higher in areas of active or recently active volcanism • Most heat is generated by radioactive decay • Maximum temperature at the center of the core is estimated to be about 6,500°C 5